1. Field of the Invention
The present invention relates to a method for separating ions with different charges of the same sign in an aqueous solution, using a charged, loose reverse-osmotic membrane (L-RO membrane). It also relates to a method for ion exchange of said substances.
2. Discussion of the Background
In the conventional technique, unnecessary ions in a solution are removed using anion- or cation- exchange resins or by crystallizing the electrolytes into salts with low solubility. With the method using an ion-exchange resin, however, separation of ions whose charge have the same sign is very difficult. Crystallization is effective only when the solubility of the electrolytic salt to be desired is low, and it is inflexible.
With the membrane method, the way in which univalent and bivalent ions in electrodialysis are affected by membrane charge is well known. It is difficult to use electrodialysis in separating univalent and bivalent ions because of the low transfer speed of these electrolytes, and it cannot be used to exchange counter-ions because they migrate in different directions according to their charges.
Separation of electrolytes using a charged ultrafiltration membrane has been studied recently. In this method, pressure is used to push the solution through the membrane, and ions are bound if they have electrolytes from non-electrolytes of identical molecular weight. This permits obtaining the charge needed to bind electrolytes in the membrane using electrolytic solutions of low concentration. This also allows separation of univalent and bivalent ions, since generally the larger the ion charge, the more effectively ions are bound. It is known that a high electrolyte concentration has no effect on the charge of the membrane and lessens the stopping effectiveness despite the ion charge. For this reason, this method of separating using a charged ultrafiltration membrane is not used.
Separation of necessary ingredients from unnecessary ones using a charged membrane is effective when separating ions in a solution which coexist with unnecessary ions of different charges and the same sign, or when the counter-ions of the desired ions coexist with unnecessary ions of different charge and the same sign. Separating electrolytes in low concentration using a charged ultrafiltration membrane is reported to be effective (See the summaries in the Autumn 19th meeting sponsored by the Chemical Engineering Society of Japan, p. 238). Use of an ultrafiltration membrane has no effect on the electric charge of the electrolyte, but shows a drop in its ability to bind electrolytes with increasing electrolytic concentration (summaries in the Autumn 19th meeting sponsored by the Chemical Engineering Society, p. 238). Consequently, an ultrafiltration membrane cannot be employed in treating electrolyte solutions in high concentrations such as fermented liquids and others which need to be separated for industry. This is true when an ion exists alone or when it coexists with other ions despite their charge, and so ions in high concentration cannot be separated by an ultrafiltration membrane.
Therefore, it has been desired to develop a method capable of separating industrially required ions from unnecessary ions even if electrolyte solutions are in relatively high concentration.